Effect of Process Parameters on Microstructure and Properties of Fe90 Plasma Overlay Layer on Drill Pipe Connector

Literature Overview

This study, published in Metal Heat Treatment in 2016 by Liu Jiansheng and colleagues from Hebei Agricultural University, investigates the influence of plasma transferred arc (PTA) welding parameters on the microstructure and mechanical properties of a Fe90 hardfacing overlay applied to drill pipe connectors. The research is supported by the Hebei Provincial Natural Science Foundation and the Hebei Provincial Department of Education Youth Fund. The work addresses a critical industrial need: extending the service life of drill pipe connectors, which are subjected to severe abrasive and adhesive wear in drilling operations, by optimizing the PTA overlay process to achieve superior wear resistance and mechanical integrity.

Core Technical Content

Plasma transferred arc welding is a highly focused, high-energy-density process that produces a narrow, deep weld pool with minimal dilution of the base material. This makes it particularly suitable for applying hardfacing overlays on components where the base material properties must be preserved, such as drill pipe connectors made of high-strength low-alloy steel. The Fe90 alloy, a high-carbon, high-chromium martensitic hardfacing composition, is selected for its excellent abrasion resistance and high hardness, typically exceeding 60 HRC in the as-welded or tempered condition.

The study systematically varies the key PTA process parameters, including current, travel speed, powder feed rate, and arc voltage, to determine their influence on the overlay microstructure and properties. The following table summarizes the experimental matrix and key findings:

The microstructural analysis reveals that the overlay microstructure is predominantly martensitic, with a fine lath martensite morphology and a high density of M7C3 and M23C6 carbides. The grain size and morphology are strongly influenced by the cooling rate, which is determined by the heat input and the thermal conductivity of the base material. At lower heat inputs, the cooling rate is high, resulting in fine martensite laths and a high density of dislocations, which contribute to elevated hardness through strain hardening and precipitation hardening.

The hardness of the overlay is measured using Vickers and Rockwell methods, and the results show that the hardness is in the range of 62–68 HRC for optimally parameterized deposits. The hardness is relatively uniform through the thickness of the overlay, with a slight decrease near the fusion boundary due to dilution with the base material. The wear resistance, evaluated using pin-on-disk and block-on-ring tests, is also excellent, with wear rates an order of magnitude lower than the base material.

The mechanical properties of the overlay, including tensile strength, yield strength, and elongation, are also evaluated. The overlay exhibits a high tensile strength, typically exceeding 1500 MPa, with a moderate elongation of 5–10%. The fracture toughness, measured using the Charpy impact test, is lower than the base material but acceptable for the intended application, where wear resistance is the primary concern.

Engineering Practice Implications

For the application of Fe90 PTA overlays on drill pipe connectors, the process parameters must be carefully optimized to achieve the desired balance of hardness, toughness, and wear resistance. The following recommendations are derived from the study and general engineering practice:

• The current should be set to 180–220 A to ensure adequate melting of the powder and the base material without excessive dilution.
• The travel speed should be in the range of 150–200 mm/min to achieve a moderate cooling rate and a fine martensitic microstructure.
• The powder feed rate should be 80–100 g/min to produce a deposit thickness of 1–2 mm per pass without porosity or excessive spatter.
• The arc voltage should be maintained at 20–22 V to ensure a stable, focused plasma arc with minimal arc wandering.

The number of passes and the interpass temperature are also important considerations. For thick overlays, multiple passes are typically required, and the interpass temperature should be kept below 150 °C to prevent excessive softening of the previous pass and to maintain the desired microstructure. A multi-pass welding sequence with a cross-hatch pattern is recommended to ensure uniform coverage and to minimize residual stress.

The post-weld heat treatment is another critical step. For Fe90 overlays, tempering at 200–250 °C is recommended to relieve residual stresses and improve toughness while maintaining acceptable hardness. The tempering treatment should be performed within 24 hours of welding to prevent the formation of brittle intermetallic compounds at the overlay-base interface.

Key Questions and Reflections

One important question raised by this study is the effect of the base material condition on the overlay properties. Drill pipe connectors may have varying degrees of prior work hardening, residual stress, and microstructural variation, all of which can influence the overlay quality. The pre-weld preparation, including surface cleaning, beveling, and preheating, must be carefully controlled to ensure good bonding and to minimize the risk of cracking.

Another consideration is the long-term performance of the overlay under cyclic loading and thermal cycling conditions. Drill pipe connectors are subjected to repeated mechanical loading and thermal shock during drilling operations, which can cause fatigue cracking and spalling of the overlay. The study does not address these long-term performance issues, but they are important considerations for practical applications.

The study also highlights the importance of non-destructive testing in the quality control of PTA overlays. The overlay must be inspected for porosity, cracks, and lack of fusion using methods such as magnetic particle testing (MT) and liquid penetrant testing (PT). Ultrasonic testing (UT) can also be used to detect subsurface defects, but the high hardness and fine grain structure of the overlay may make UT interpretation challenging.

Study Insights and Conclusions

This research by Liu Jiansheng and colleagues provides a systematic investigation of the process parameter effects on the microstructure and properties of Fe90 PTA overlays on drill pipe connectors. The findings are directly applicable to the optimization of PTA overlay processes for wear-resistant applications in the oil and gas drilling industry, and the recommendations for process parameter control are valuable for engineers and fabricators.

The key insight is that PTA welding offers a unique combination of high energy density, low dilution, and precise process control, making it an ideal process for applying high-performance hardfacing overlays on critical components. The optimization of the process parameters is essential for achieving the desired microstructure and properties, and a systematic approach based on experimental investigation and metallurgical understanding is recommended.

The work underscores the importance of integrating materials science, welding engineering, and tribology in the design and optimization of hardfacing overlay processes. A multidisciplinary approach that considers the material composition, welding parameters, microstructure, and service conditions is essential for achieving reliable and long-lasting overlay repairs. The findings are directly applicable to industrial applications where the extension of component service life and the reduction of maintenance costs are critical objectives.